Electrical machines (motors and generators) are widely used to process energy and power equipment. Many of their applications require the motors and generators to operate at variable speed. Power electronics equipment is also used to operate with the motors and generators in such variable speed systems, including but not limited to industrial drives, electrical vehicles, diesel-generator sets, and wind power generation. There is a strong desire to increase the efficiency of such systems, while reducing its cost and size, especially for demanding applications such as electrical cars and other transportation equipment.
Unfortunately, the motor, generator and power electronics equipment in variable speed systems usually adopted standard technologies in each area, and are usually not optimized as a whole to achieve best results. For example, so far the vast majority of the motors and generators have a three-phase structure, and the power converters working with them are of a three-phase structure too. Significant improvement is needed to further optimize system performance and reduce the system cost.